The present invention concerns a method for adjusting a pressure sensor in an SCR system during a switch-off phase. The invention further concerns a computer program that carries out each step of the method when run on a computing device and a machine-readable memory medium that stores the computer program. Finally, the invention concerns an electronic control unit that is arranged to carry out the method according to the invention.
Methods and devices are known for operating a combustion engine, in particular in motor vehicles, in the exhaust region of which an SCR catalytic converter (Selective Catalytic Reduction) is disposed that reduces the oxides of nitrogen (NOx) contained in the exhaust gas of the combustion engine to nitrogen in the presence of a reagent. This enables the proportion of oxides of nitrogen in the exhaust gas to be considerably reduced. DE 103 46 220 A1 describes the basic principle. Ammonia (NH3) is required for the reaction process. As the reduction agent, reagents that separate NH3 are used, which are admixed with the exhaust gas. As a rule, for this purpose a 32.5% urea-water solution (HWL), also known commercially as AdBlue®, is used, which is metered into the exhaust gas system upstream of the SCR catalytic converter.
The reduction agent solution is usually stored in a reduction agent tank in the motor vehicle. For delivery and metering of the urea solution, in general a dosing system is provided comprising a delivery pump, a pressure line, a dosing module with a dosing valve, as well as the necessary sensor arrangement and an electronic control unit. The delivery pump transports the urea solution from the reduction agent tank via the pressure line into the dosing module. For dosing according to demand, the desired and necessary mass and/or volume of the reduction agent solution is metered into the exhaust gas system via the dosing valve.
Typically, a pressure sensor is used to monitor the system pressure. In order to ensure correct operation of said pressure sensor, the pressure measured by the pressure sensor is compared with a reference pressure, for example the ambient pressure.
DE 10 2012 214 369 A1 concerns an exhaust aftertreatment arrangement and a method for metering a reduction agent into the exhaust gas system of a combustion engine with a reduction agent tank. In this case, the pressure sensor detects a reduction agent pressure and compares the pressure measured by the pressure sensor with a reference pressure, for example the ambient pressure. By means of said comparison, when the combustion engine is started and with the reduction agent tank empty it can be determined whether the pressure sensor is operating properly. In a first step, by using the pressure sensor, when the combustion engine is started the reduction agent pressure can be compared with the ambient pressure. If the measured reduction agent pressure differs from the ambient pressure, then a malfunction of the pressure sensor can be concluded. It is therefore provided that the comparison with the ambient pressure is carried out with the combustion engine switched off in order to carry out an adjustment of the pressure sensor in the event of a restart of the combustion engine.
At low temperatures, the reduction agent solution freezes. For example, the freezing point of the frequently used solution AdBlue® is at approx. −11° C. Therefore, measures are provided in a switch-off phase of the SCR system to protect the SCR system at low temperatures against damage because of the freezing of the reduction agent solution. Typically, in the switch-off phase the reduction agent solution is returned into the reduction agent tank by means of the delivery pump or a return pump. In this case, the delivery pump or the return pump builds up a reduced pressure in the pressure line. By opening the dosing valve, the pressure line is ventilated and the reduction agent solution is sucked into the reduction agent tank.
DE 10 2014 202 038 A1 concerns a method for operating an SCR system in a motor vehicle. In particular, the method concerns such return suction or return transport of the reduction agent solution when the motor vehicle is turned off.